Hydrocarbons, and specifically olefins such as ethylene, are typically building blocks used to produce a wide range of products, for example, break-resistant containers and packaging materials. Currently, for industrial scale applications, ethylene is produced by heating natural gas condensates and petroleum distillates, which include ethane and higher hydrocarbons, and the produced ethylene is separated from a product mixture by using gas separation processes.
Oxidative coupling of the methane (OCM) has been the target of intense scientific and commercial interest for more than thirty years due to the tremendous potential of such technology to reduce costs, energy, and environmental emissions in the production of ethylene (C2H4). As an overall reaction, in the OCM, CH4 and O2 react exothermically over a catalyst to form C2H4, water (H2O) and heat.
Ethylene can be produced by OCM as represented by Equations (I) and (II):2CH4+O2→C2H4+2H2O ΔH=−67 kcal/mol  (I)2CH4+½O2→C2H6+H2O ΔH=−42 kcal/mol  (II)
Oxidative conversion of methane to ethylene is exothermic. Excess heat produced from these reactions (Equations (I) and (II)) can push conversion of methane to carbon monoxide and carbon dioxide rather than the desired C2 hydrocarbon product (e.g., ethylene):CH4+1.5O2→CO+2H2O ΔH=−124 kcal/mol  (III)CH4+2O2→CO2+2H2O ΔH=−192 kcal/mol  (IV)The excess heat from the reactions in Equations (III) and (IV) further exasperate this situation, thereby substantially reducing the selectivity of ethylene production when compared with carbon monoxide and carbon dioxide production.
Additionally, while the overall OCM is exothermic, catalysts are used to overcome the endothermic nature of the C—H bond breakage. The endothermic nature of the bond breakage is due to the chemical stability of methane, which is a chemically stable molecule due to the presence of its four strong tetrahedral C—H bonds (435 kJ/mol). When catalysts are used in the OCM, the exothermic reaction can lead to a large increase in catalyst bed temperature and uncontrolled heat excursions that can lead to catalyst deactivation and a further decrease in ethylene selectivity. Furthermore, the produced ethylene is highly reactive and can form unwanted and thermodynamically favored deep oxidation products.
Generally, in the OCM, CH4 is first oxidatively converted into ethane (C2H6), and then into C2H4. CH4 is activated heterogeneously on a catalyst surface, forming methyl free radicals (e.g., CH3.), which then couple in a gas phase to form C2H6. C2H6 subsequently undergoes dehydrogenation to form C2H4. An overall yield of desired C2 hydrocarbons is reduced by non-selective reactions of methyl radicals with oxygen on the catalyst surface and/or in the gas phase, which produce (undesirable) carbon monoxide and carbon dioxide. Some of the best reported OCM outcomes encompass a ˜20% conversion of methane and ˜80% selectivity to desired C2 hydrocarbons.
There are many catalyst systems developed for OCM processes, but such catalyst systems have many shortcomings. For example, conventional catalysts systems for OCM display catalyst performance problems, stemming from a need for high reaction temperatures. Thus, there is an ongoing need for the development of catalyst compositions for OCM processes.